Antenatal Group B streptococcus screening

1.What is Group B streptococcus (GBS)?

  • GBS is a commensal bacterium found in the bowel or vagina of healthy adults. The colonisation can be transitory, intermittent or persistent;
  • Colonisation at these sites do not cause symptoms and usually requires no treatment;
  • In a local study involving over 110,000 pregnant women in Hong Kong, GBS colonisation rate is 21.8% (Ma TW et al 2018)

2.Implication of GBS vaginal colonisation

  • The baby may acquire the infection following labour or after rupture of membranes in 1-2% of cases;
  • GBS is the commonest cause of severe early onset infection in newborn with a high rate of morbidity and mortality (5-10%)


3. Protocol of GBS screening in Hong Kong
A routine low vaginal and rectal swab is taken from pregnant women at 35-37 weeks of gestation unless the women are already known to be carriers of GBS. The swab result is representative for the next 5 weeks and hence the status when in labour. The test should be repeated if the women have not delivered after 5 weeks.

Prophylactic intravenous antibiotics to the mother during labour period is indicated for the followings:

  • Positive GBS culture in any vaginal swab during the antenatal period;
  • Positive GBS culture in any urine culture during the antenatal period;
  • Neonatal GBS infection in the previous pregnancy

Prophylactic intrapartum intravenous antibiotics has been shown to reduce the chance of early onset neonatal GBS infection to < 0.4/ 1,000 deliveries

4. Any need for treatment of GBS vaginal colonisation in the antenatal period?
Instantaneous antibiotics treatment after a positive vaginal culture in the antenatal period fails to eradicate carriage of GBS in vagina or to prevent neonatal infection. It is therefore not recommended by the Centre of Disease Control in USA and the American College of Obstetricians and Gynecologists.

However, positive GBS culture from the urine sample warrants treatment for the urinary tract infection.


Prevention of Group B streptococcal early-onset disease in newborns (ACOG Committee Opinions Feb 2020)

Ma TWL, Chan V, So CH, Hui ASY, Lee CN, Hui APW, So PL, Kong CW, Fung B, Leung KY. Prevention of early onset group B streptococcal disease by universal antenatal culture-based screening in all public hospitals in Hong Kong. J Matern Fetal Neonatal Med. 2018 Apr;31(7):881-887.

This article is contributed by Dr. T.N. Danny Leung
updated on 25.09.2020